kumarUjjawal opened a new pull request, #19926:
URL: https://github.com/apache/datafusion/pull/19926
## Which issue does this PR close?
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- Closes #19921.
## Rationale for this change
`SELECT ROUND('999.9'::DECIMAL(4,1))` incorrectly returned `100.0` instead
of `1000`.
When rounding a decimal value causes a carry-over that increases the number
of digits (e.g., 999.9 → 1000.0), the result overflows the original precision
constraint. The overflow was silently truncated during display, producing
incorrect results.
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## What changes are included in this PR?
- Increased the precision by 1 for all decimal types (capped at max
precision)
- Added and udpated slt tests
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## Are these changes tested?
- Added new sqllogictest case for the specific bug scenario
- Updated 2 existing tests with new expected precision values
- All existing tests pass
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## Are there any user-facing changes?
| Aspect | Before | After |
|--------|--------|-------|
| `ROUND('999.9'::DECIMAL(4,1))` | `100.0` (wrong) | `1000` (correct) |
| Return type | `Decimal128(4, 1)` | `Decimal128(5, 1)` |
The return type precision is now increased by 1 to prevent overflow. This
matches PostgreSQL behavior where `ROUND` can increase the number of digits
before the decimal point when carry-over occurs.
Users who depend on exact output precision may need to explicitly cast:
```sql
SELECT CAST(ROUND(...) AS DECIMAL(p, s))
```
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